1. Field of the Invention
The present invention relates to a stage device for supporting and two-dimensionally moving a workpiece such as a semiconductor wafer, equipped with a mechanism for levelling said workpiece.
2. Related Background Art
A particularly important step in the process for working a semiconductor wafer is a printing step, called photolithography. There have been employed various exposure apparatus in this step, but a reduction-projection exposure apparatus, or so-called stepper, is being principally utilized in order to meet the rapid progress in the degree of integration and toward a finer line width. This apparatus projects a pattern formed on a photomask, called reticle, through a projection lens onto photoresist coated on a wafer, thereby exposing said photoresist to the light. Since the field of each exposure is smaller than the size of the wafer, the exposures are conducted by step-and-repeat process in which a stage supporting the wafer is two-dimensionally moved. In such steppers a higher resolving power is required in the projection lens year after year, and a higher numerical aperture of the lens and a larger exposure field are simultaneously desired. However a projection with a large numerical aperture and with a large exposure field is inevitably associated with a smaller depth of focus, so that the margin for exact focusing over the entire exposure field is considerably small.
The surface of the wafer to be exposed is seldom set constantly parallel to the moving plane of the wafer supporting stage, or the imaging plane of the pattern of the reticle. In fact the wafer surface may be inclined in excess of the depth of focus of the projection lens on both sides of the exposure field, due to incomplete flatness of the wafer chuck and of the wafer itself. A stepper equipped with so-called wafer levelling mechanism, for finely adjusting the inclination of wafer on the stage, in order to correct the above-mentioned inclination, is already disclosed for example in the U.S. Pat. No. 4,383,757. Such already known wafer levelling mechanism is provided with so-called through-the-lens (TTL) levelling sensor for determining the axial aberrations between the image plane and the wafer surface at the plural points in the exposure field through the projection lens and detecting the amount and direction of inclination from said aberrations, and is adapted to level the sensor based on the detection signal from said sensor.
Such conventional mechanism may however result in an alignment error (error in the registration between the projected image of the reticle pattern and the area to be exposed on the wafer) after the wafer is levelled. This is not a problem if the TTL alignment through the projection lens is conducted simultaneously with the TTL levelling, as each exposure can be achieved without the error in alignment in this manner. However the necessity of alignment for each exposure reduces the throughput. Also the TTL levelling sensors currently developed may not be ideal for such detection as they are often strongly influenced by the surface state of the wafer.
In contrast to the step-and-repeat exposure process with alignment for each exposure, there is also a process of moving the wafer stage to a position predetermined by designing after the global alignment of the wafer and conducting exposure at said position. This process can achieve a highest throughput and is suitable for the mass production of IC's such as semiconductor memories. In this process, for levelling the wafer without sacrificing the throughput, it is desirable to dispense with the alignment for each exposure. For this purpose the mechanism has to be so constructed as to avoid lateral movement of the wafer in the levelling operation after the stepping motion of the wafer stage for an exposure, but the conventional mechanisms cannot necessarily ensure the absence of such lateral movement.